In the production of hydrocarbon from wellbores, it is common to employ a pumpjack unit for actuating a string of sucker rods which reciprocates a bottomhole pump. Each cycle of the pumpjack unit imposes varying loads on the pump and rodstring and accordingly, the load on the motor continually varies in a cyclic manner each reciprocation of the pumpjack as shown in FIG. 6 of the drawings.
FIG. 6 shows one complete reciprocation of a rod string of a pumpjack unit, referred to herein as an upstroke and a downstroke of the pumpjack unit. As evidenced in the above pumpstroke cycle, the motor load varies from 10% up to 200% of the motor rated current under some conditions of operation. Hence, the oilwell pumpjack load is unique. The starting torque required for actuating the massive weights of the rod string and fluid column or counter balance weights is considerable, and as the pumpjack strokes, the load changes generally in the manner shown in FIG. 6. Hence, each cycle of operation or stroke, has a rod peak, a weight peak, and two off-peaks. The frequency of the cycle, or stroke, depends on the strokes per minute that the pump is reciprocating, and is, generally, in the range of 4-10 seconds in duration. If the well is stroked ten strokes per minute by the pumpjack unit, a complete cycle would occur each six seconds. And during that time, the current through the motor can vary from 10-20% of the rated motor current for off-peak readings and to as much as 180%-200% of the rated motor current for either or both of the rod and weight peak currents.
For this reason, the high slip and, later, the ultra high slip motor was developed and constructed so that the motor could slip, or drag down, such that it varies in speed as a result of changing torque demands during each cycle of the pumpjack apparatus. Hereafter, these motors shall be referred to as "oilwell pumping motors".
Most oilwell pumping motors utilized in conjunction with pumpjack units are 440 volts, three phase A.C. and range from ten to one hundred twenty-five horsepower. These specifically designed oilwell pumping motors have the three field coils or stators thereof each comprised of dual windings so that nine leads exit from the motor, thereby enabling the windings to be connected in Y, .DELTA.Y, or .DELTA. configuration, respectively; thus providing a selection of low, medium, and high running torques, respectively.
Upon initial installation of the oilwell pumping motor, a selection of the electrical connection of the nine electrical leads leading to the various windings must be mechanically effected at that time, and should it subsequently be desired to change the winding connections from .DELTA. to .DELTA.Y, for example, thereby reducing the available torque or power, it is necessary for the electrician to visit the well site, break the old connections, and make the necessary changes. Should these changes from .DELTA. to .DELTA.Y subsequently prove to be more than adequate for lifting the production fluid, the motor will run under unnecessarily high torque conditions. There still remains the unused and more desirable low torque Y connection, which is the most desirable mode of operation when the pumpjack load conditions permit this configuration.
Accordingly, anytime the motor torque mis-matches the requirements of the pumpjack unit, it is necessary to recognize the undesirable condition and to instruct an electrician to mechanically effect the required changes in the motor winding, or otherwise the motor will not be operating in its most efficient torque mode and, in an extreme example, the motor would overheat and shut down if overloaded.
Accordingly, it would be desirable to have made available a means by which the field windings of an oilwell pumping motor is automatically connected to provide the torque best suited for the particular hydrostatic head and load associated with the particular wellbore which it is producing. An automatic load seeking control which provides this selection is the subject of the present invention.